DE69628406T2 - Generation of mechanical force by expansion of liquid into vapor - Google Patents

Description

The invention relates to a method and a device for extracting mechanical power the expansion of a liquid or a wet steam to steam by means of a displacement machine.

The term extrusion machinery used here refers to a machine or a series of two or more Machines in which or with each of them at least one chamber for Collecting a working fluid cyclically goes through the following steps: a filling of the working fluid, closing, enlarging or reducing thereof Volume, opening for dispensing the working fluid filling and then decrease or increase of their volume, namely to the value at the beginning of the cycle was achieved. The built-in volume expansion ratio, like it used herein with respect to one as an expansion device displacement machine is the ratio of the maximum volume of a working chamber just before opening their volume at the time the chamber is closed.

If the machinery consists of two or several displacement machines , which are arranged in series, the built-in volume expansion ratio is the Machinery the product of built-in volume expansion ratios of the individual machines.

It is well known that most Displacement machines, which are used as compressors, also in reverse as Expansion devices can work to provide mechanical power output to create. Consequently, double screw, single screw, Spiral, shovel and piston machines all suggested for or on top of them Operated wisely.

However, in many cases, though such machines worked as expansion devices, clearly achieved lower efficiencies than those expected especially when used to expand saturated liquids or wet vapors were. U.S. Patent No. 3,751,673 (Sprankle) describes the concept of using a Lysholm twin screw machine for expanding hot water under pressure in the form of geothermal brines. In this case, despite a larger research program maximum adiabatic efficiencies of just over 50% reached while values up to 75% were expected, see J. Kestin, "Sourcebook on the production of electricity from geothermal energy ", DOE / RA / 28320-2, August 1982; R.F. Steidel, H. Weis and J.E. Flower, "Characteristics of the Lysholm engine as tested for geothermal applications in the Imperial Valley ", J Eng for Power, v 104, pp. 231-240, January 1982; and R.J. LaSala, R. McKay, P.A. Borgo and J. Kupar, "Test and Demonstration of 1 MW Wellhead Generator: Helical Screw Expander Power Plant, Model 76-1, "Report to the International Energy Agency. DOE / CE-0129 U.S. Department of Energy Div of Geothermal and Hydropower Technology, Washington, D.C. 20585, 1985.

European Patent Application No. 0 082 671 describes a method and an apparatus for winning of mechanical power from the expansion of a working fluid from a first pressure to a second, lower pressure, where the device is a displacement machine having. According to one In the first aspect, the invention provides a device as described in Claim 1 is claimed. According to one second aspect, the invention provides a method as claimed 3 is claimed.

Water is excluded from its very high volume expansion ratio from the liquid state to the vaporous Condition that at normal power plant condensation temperatures the size of several Thousand lies.

The invention is exemplified with Further described with reference to the accompanying drawings, where:

1 Fig. 10 is a sectional graphical view of a vane type compressor;

2 Fig. 12 is a graph showing the change in pressure with the change in volume of a working chamber of a normal compressor in normal operation;

3 and 4 are closed 2 corresponding graphs showing the effects of applying too high or too low pressure in a compressor;

5 Fig. 12 is a graph showing the expected performance of a prior art displacement machine used to expand a liquid;

6 Figure 3 is a graph showing the actual performance achieved by the prior art;

7 Figure 3 is a graph showing the performance achieved using the invention;

8th Fig. 3 is a schematic diagram of a cooling or refrigeration system to which the invention can be applied;

9 shows a modification of 8th ;

10 Figure 3 is a schematic circuit diagram of a heat pump incorporating the invention;

11 Fig. 3 is a schematic circuit diagram of a plant for generating electricity from a low quality heat source such as a geothermal brine; and

12 is a temperature graph that shows the entropy of the work cycle of 11 is drawn.

1 schematically shows a conventional vane type compressor as an example of a displacement machine. Other examples are the Lysholm screw machines mentioned above, single screw compressors, constrained-vane compressors, reciprocating-cylinder machines. The compressor shown has a stator housing 1 with a cylindrical interior 2 with one axis 3 , a smaller opening 4 , which forms the outlet of the compressor, and a larger opening 5 that forms the inlet.

A cylindrical rotor 6 with a smaller diameter than the interior 2 is attached to it around an axis 7 to rotate that with the axis 3 is parallel but is spaced from it. blades 8th can be moved in equidistant pockets 9 are arranged in the rotor and are pressed outwards as the latter rotates in order to make sealing contact with the inner wall of the housing and thus the space between the rotor 6 and the housing 1 into a set of work chambers 10a to 10h split, the volume of each of these from a minimum between positions 10a and 10b to a maximum between positions 10e and 10f varied. The rotor turns in the direction of the arrow 11 driven when used as a compressor. When used as an expansion device, the opening forms 4 the inlet and the opening 5 the outlet, and rotates the rotor in the opposite direction.

When used as a compressor, the processes involved in gas or steam compression follow that in 2 shown path. The working fluid is introduced at an approximately constant pressure (range PQ) at a value which is slightly lower than that in the inlet opening or a distributor device 5 , followed by compression (area QR) to the desired discharge pressure by volume reduction (RS) and then discharge under an approximately constant pressure that is slightly higher than that in the feed manifold. The pressure differences between the inside and the outside of the machine during suction and discharge are relatively small and can be neglected in a first approximation.

The mass flow rate through the machine is mostly by the volume swept by the machine certainly. In practice, the true induced volume is slightly lower than the swept value due to a backward leakage flow of the Fluids between the blades, the rotors or the piston and the casing in the filling volume, what by the pressure gradient, by the compression process is generated, induced. This difference is called volume efficiency or as a relationship of the induced fluid volume to the swept volume in the Machine during the filling process expressed. For screw type compressors where the free space is negligible this can be at the size of 95% lie.

For compressors, the built-in volume expansion ratio can be selected approximately as the value needed to increase the pressure from a suction value to a discharge value according to the pressure-to-volume relationship appropriate for the assumed compression process, ie with or without liquid injection or external heat transfer. If the assumed value is not correct, there is either too much pressurization of the fluid at position (R) in the compression process where the dispensing process begins, as in FIG 3 is shown, or pressurization too low, as in 4 is shown. In both cases, the impact on compressor performance and efficiency is relatively low.

In the case of displacement machines used as expansion devices, it would be a reasonable starting point for the design to assume that the sequence of processes involved is roughly the reverse of that of the compressor, as shown in 5 is shown. However, a more detailed analysis in connection with the invention has shown that the path of the processes in an expansion device differs significantly from that in a compressor, especially when the working fluid enters the machine as a saturated liquid or a wet vapor, and that the optimal value of the built-in volume expansion ratio differs significantly from that which is required to bring about the reverse pressure change in a compressor than could reasonably be expected from a simplified analysis. In addition, the selection of the wrong value has profound effects on the size, speed and efficiency of the expansion device.

The differences between the processes derived from the assumption of an inverted compressor are shown in 5 shown and can from 6 and 7 be inferred. As you can see, the deviations are big. The reasons for this can be explained as follows:
First, it can be seen that the filling process TU is associated with a significant pressure drop and consequent expansion. This happens because the fluid, which is accelerated through an inlet opening, receives kinetic energy. This increase in kinetic energy is much greater with wet fluids than with gases, since the wet fluids are much denser.
Secondly, since the fluid is admitted to the end of the machine at the high pressure, the induced mass of fluid strongly depends on the built-in volume expansion ratio and increases as the volume ratio decreases. In this case, leakage flows in the same direction as the fluid flow. As a result, the fluid expands through the clearances between the blades, the rotors, or the pistons and the housing even when the rotor of the expansion device is not rotating or the pistons are not being displaced. Consequently, the induced volume flow rate is higher than the volume swept by the blades, rotors or pistons during the filling process. The leak rate mainly depends on the clearances between the blades, the rotors or the pistons and the housing and is largely independent of the built-in volume expansion ratio and the built-in volume expansion speed. It follows that if the built-in volume expansion ratio is reduced, the leakage flow becomes a smaller part of the total flow and consequently its influence on the machine performance is reduced.

While the speed of the expansion device is increased, will be additional the kinetic energy increase of the fluid when flowing through the inlet opening is greater. consequently the pressure drop and expansion increase with the filling process are connected. The density of the induced fluid is therefore reduced and therefore the mass flow through the machine does not increase as quickly, how to increase the machine speed would expect.

In 6 the expansion in the filling process is so great that the fluid has been overexpanded (YV) before the dispensing process begins. This overexpansion can be caused either by operating the expansion device at too high a speed or by a too large built-in volume expansion ratio. Whatever the cause, it can clearly be seen that it reduces the area of positive work (P) and also creates a large area of negative work (N) in the pressure-volume diagram and consequently has a double adverse effect on the Has efficiency of the expansion device, which is much larger than an equivalent, too high pressurization with a compressor.

In 7 Using the invention, it can also be seen that at the end of the UV expansion process, the fluid pressure is greater (above VW) than during the dispensing process (WX). As a result, there is a small loss of potential work due to under-expansion (right of VW).

Another characteristic that is performance of all displacement machines affects whether they are in the expansion mode or in the compressor mode working is internal friction. In all cases, the associated increases Loss of efficiency with speed. The best construction an expansion device is therefore a compromise between the need for high speed to avoid leakage minimize, and a low speed to reduce friction minimize a big one built-in volume expansion ratio to make up for losses of under expansion, and a small volume ratio to decrease to minimize the importance of leakage effects during the Mass flow is maximized and thereby the size of the expansion device is kept to a minimum.

The optimal built-in volume expansion ratio

Because you look at both performance losses through overexpansion as well as underexpansion, one could conclude that The best results are obtained when the pressure at the end of the Expansion exactly what is needed Discharge pressure corresponds. In practice, for a given machine size, there is that working at a specified speed, a choice between with a low built-in volume expansion ratio a consequently high induced mass flow and relatively low leakage losses, but some losses due to under-expansion, and a higher built-in Volume expansion ratio with a lower induced mass flow rate and higher leakage losses, however little or no loss due to under-expansion. An increase the speed increases the internal expansion of the filling process and thus allows an even lower built-in volume expansion ratio, however elevated the friction losses. If all of these effects are considered at the same time , it has been found that if there is some underexpansion is allowed designs for high speed and one low built-in volume expansion ratio, which is lower than that for Alternatives with a complete Low speed expansion needed the highest adiabatic Achieve overall efficiency. If the impact of expansion while the filling process is included, it becomes clear that the built-in volume expansion ratio required for an optimal Size and one optimal efficiency is much lower than that of the overall expansion process from entering the machine to exiting the machine.

Numerical values for this Difference one when you are replacing a throttle valve in two large industrial cooling units either by a single screw expansion device or by considered a two-screw expander to mechanical Generate power.

In the 8th Chiller installation shown is conventional in that it has a drive motor M whose shaft 21 a compressor for compressing refrigerant vapor from an evaporator 23 drives the heat from a cooling circuit 24 dissipates. The compressor 22 delivers hot, compressed steam to a condenser 25 where it passes through a heat exchanger, the liquid in a cooling circuit 26 has, cooled to a liquid and condensed.

Traditionally, the liquid coolant would reduce its pressure by passing through a throttle valve 27 flows, but instead it is here through a two-phase expansion device 28 expanded according to the invention (from liquid to vapor). The output of the expansion facility 28 is on a wave 29 applied to the motor M either directly or via a gearbox when driving the compressor 22 to support.

The total volume ratio of one Expansion of a compressor can be called the ratio of the specific volume at the outlet to the specific volume at the inlet of the compressor can be defined, where the specific volume is the volume of a Is substance per unit of mass.

9 shows a modification of 8th , in which the two-phase expansion device 28 is arranged to a second steam compressor 30 to drive that to the main compressor 22 is connected in parallel. Both the expansion device 28 as well as the second steam compressor 30 are of the Lysholm double screw type. Using coolant 134A as the working fluid leads to the following results:

In both cases it was estimated that the adiabatic efficiency of the expansion devices over 75% are.

In addition to replacing the throttle valve in industrial refrigeration machines, positive displacement expansion devices can be used for the same function in large heat pumps and cold stores in an identical or similar manner, as for example in FIG 10 is shown.

At the in 10 The arrangement shown is that the main compressor is a two-stage compressor, which is a low-pressure compressor driven by a motor M1 41 includes its delivery through a line 42 to the inlet of the high pressure compressor 43 the second stage is delivered. Delivery of the capacitor 25 flows through a throttle valve 44 for partial expansion into a vapor / liquid separation device 45 from which the steam flows through a pipe 46 to the management 42 is delivered to the inlet of the high pressure compressor 43 provided.

The liquid of the separation device 45 is at the inlet of the expansion device 28 supplied, the outlet of which is at the inlet of the evaporator 23 connected. The output shaft 46 the expansion device is connected to a two-stage compressor 47 to drive, which consists of two screw compressors connected in series, which act as a low pressure stage 48 and a high pressure stage 49 are constructed. The low pressure stage receives from the evaporator outlet via a line 50 Steam, and the delivery of the high pressure stage 49 is through a line 51 to the inlet of the condenser 25 delivered.

The circle 26 is by extracting heat from the circle 24 circuit to be heated when used as a heat pump.

Such machines can also be used as the main expansion device in a system for recovering energy from low quality heat sources, such as geothermal brines, which the inventors refer to as the trilateral flash cycle (TFC) system. In 11 the circle is shown and its cycle is in 12 shown. In this case the temperature changes and consequently the volume ratios are much larger and consequently two or more expansion stages are needed which work in series. A typical example of this is how it is in 11 is shown the case of supplying a hot brine in the form of a saturated liquid with a temperature of 150 ° C., which is currently separated from wet steam in a rapid evaporation plant and is returned to the soil at this temperature. A study showed that by passing the brine from a line using countercurrent heat exchange with the working fluid through a TFC primary heat exchanger 51 it can first be cooled to 45 ° C along path AB before being reintroduced, and an energy of 3.8 MW could be obtained from the heat extracted from it. In this case the working fluid in the system is n-butane with a temperature at the inlet of the expansion device 52 of 137 ° C and a condensation temperature in a condenser 53 of 35 ° C, its condensate from a feed pump 54 is pressurized and to the heat exchanger 51 is delivered back. A large two-stage twin screw expansion system (which drives a generator G) was considered the most suitable for this purpose, the main features of which are:

As a result, a total volume expansion ratio of 34.8: 1 for two stages with a built-in total volume ratio of only reached 11.5: 1, resulting in a relative ratio of 11.5 / 34.8 = 33% leads.

Taking inherent reheating effects into account for multi-stage expansion devices, the overall adiabatic efficiency is this expansion arrangement 82.2%, the value well with the one Dry steam turbine of equivalent power output matches. It is found that such high levels of efficiency cannot be achieved with screw compressors can be obtained the directly above same Pressure differences work and that the net system levy by 35% is higher than those for an alternative conventional power plant that works for the same Function is offered, is asserted.

Claims (4)

Device for deriving mechanical power from the expansion of a working fluid, other than water, from a liquid state at a first pressure to steam at a second, lower pressure, the device comprising a displacement machine ( 1 ; 28 ), characterized in that the built-in volume expansion ratio of the displacement machine ( 1 ; 28 ) is between 10 and 50% of the total volume expansion ratio that the fluid experiences during the pressure reduction between the entry into the machine and the exit from the machine. The device of claim 1, wherein the built-in relationship is between 20 and 40% of the total expansion ratio. A method of deriving mechanical power from the expansion of a working fluid, other than water, from a liquid state at a first pressure to steam at a second, lower pressure, the method using a positive displacement machine ( 1 ; 28 ), characterized in that the built-in volume expansion ratio of the displacement machine ( 1 ; 28 ) is between 10 and 50% of the total volume expansion ratio of the fluid in the pressure reduction between the entry into the machine and the exit from the machine. The method of claim 3, wherein the built-in ratio between Is 20 and 40% of the total expansion ratio.